Polyethylene process

Abstract

An ethylene polymerization process is disclosed. Ethylene is polymerized in two slurry reaction zones with a C 6 -C 10 alpha-olefin in the presence of a single-site catalyst capable of making a high molecular weight polyolefin. The process yields medium density and linear low density polyethylene having a bimodal molecular weight distribution and a melt index from about 0.10 to about 0.80. Films from the polyethylene have superior impact properties.

1. A process which comprises polymerizing ethylene with a C 6 -C 10  alpha-olefin in two slurry reaction zones in the presence of a single-site catalyst system comprising an activator and a supported Group 4-6 transition metal complex to produce polyethylene, wherein from about 35 to about 75 wt. % of the polyethylene is produced in a first reaction zone with a hydrogen to ethylene molar ratio in the vapor phase of from about 0.00005:1 to about 0.01:1 to produce a first-zone material having a melt index of from about I to about 300 dg/min which is transferred as a slurry to a second reaction zone having a hydrogen to ethylene molar ratio in the vapor phase of from about 0.000001:1 to about 0.003:1 to produce polyethylene with a bimodal molecular weight distribution, a density of from about 0.91 to about 0.94 g/cm 3 , and a melt index of from about 0.10 to about 0.80 dg/min, wherein each of said melt indices is measured according to ASTM D1238, Condition 190/2.16. 
     
     
       2. The process of  claim 1  wherein the polyethylene, when blown in-the-pocket into a film having a thickness of 13 microns, has a dart drop impact strength of greater than about 100 grams. 
     
     
       3. The process of  claim 2  wherein the dart drop impact strength is greater than about 140 grams. 
     
     
       4. The process of  claim 1  wherein the C 6 -C 10  α-olefin is 1-octene. 
     
     
       5. The process of  claim 1  wherein greater than 50% by weight of the total C 6 -C 10  α-olefin is added to the second reaction zone. 
     
     
       6. The process of  claim 5  wherein greater than 80% by weight of the total C 6 -C 10  α-olefin is added to the second reaction zone and the C 6 -C 10  α-olefin is 1-octene. 
     
     
       7. The process of  claim 5  wherein each zone is operated at a temperature of from about 60° C. to about 100° C. 
     
     
       8. The process of  claim 1  wherein the polyethylene has a density of from about 0.915 to about 0.925 g/cm 3 . 
     
     
       9. The process of  claim 8  wherein the polyethylene has a density of from 0.920 to 0.925 g/cm 3 . 
     
     
       10. The process of  claim 1  wherein the transition metal complex contains at least one bridged indenylindolyl ligand. 
     
     
       11. The process of  claim 1  wherein the transition metal complex incorporates a Group 4 transition metal and at least one aryloxy ligand.